Safe, efficient and economical operation of a motor vehicle depends, to a significant degree, on maintaining correct air pressure in all (each) of the tires of the vehicle. Operating the vehicle with low tire pressure may result in excessive tire wear, steering difficulties, poor road-handling, and poor gasoline mileage, all of which are exacerbated when the tire pressure goes to zero in the case of a “flat” tire.
The need to monitor tire pressure when the tire is in use is highlighted in the context of “run-flat” or extended mobility tires, tires which are capable of being used in a completely deflated condition. Such run-flat tires, as disclosed for example in U.S. Pat. No. 5,368,082, incorporated herein by reference, may incorporate reinforced sidewalls, mechanisms for securing the tire bead to the rim, and a non-pneumatic tire within the pneumatic tire to enable a driver to maintain control over the vehicle after a catastrophic pressure loss, and have evolved to the point where it is becoming less and less noticeable to the driver that the tire has become deflated. The broad purpose behind using run-flat tires is to enable a driver of a vehicle to continue driving on a deflated pneumatic tire for a limited distance prior to repair of the deflated tire. Hence, it is generally desirable to provide a low tire pressure warning system within the vehicle to alert the driver by way of a light or an audible alarm to the loss of air pressure in a pneumatic tire. In a general sense, such a warning system may also be utilized to identify the particular tire experiencing the loss of air pressure and trigger automatic means for tracking the mileage that the tire has been driven in the deflated condition.
To this end, a number of electronic devices and systems are known for monitoring the pressure of pneumatic tires and providing the operator of the vehicle with either an indication of the current tire pressure or alerting the operator when the pressure has dropped below a predetermined threshold level. It is also known to monitor tire pressure with an electronic device that is not merely a passive resonant circuit, but rather is capable of transmitting a radio frequency (RF) signal indicative of the tire pressure to a remotely-located receiver. Such a “transmitting device” may have its own power supply or, alternatively, may be activated by an RF signal from the remote receiver. In the latter form, the transmitting device is said to be “passive”.
A typical tire monitoring system will have a receiver with one or more antennas for receiving signals from the tags in each of the tires on the vehicle. The tags may be packaged and mounted within the tire cavity or, alternatively mounted to the valve stem in communication with the tire cavity. In order to report tire conditions-that are properly identified with each of the tires on the vehicle, the monitoring system receiver must be able to determine from which tire the received RF signals originates. It is known, accordingly, to incorporate within each tag a unique identifying code (ID) and to configure the tag to include its ID within a data stream transmitted by the tag.
Various means for measuring air pressure are utilized within known tire pressure monitoring systems. It is known, as in U.S. Pat. No. 4,578,992, to use a pressure-sensitive capacitor forming a passive oscillatory circuit having a resonant frequency which varies with tire pressure. Other suitable pressure transducers known in the art include piezoelectric devices; silicon capacitive pressure transducers; variable-conductive laminates of conductance ink; and devices formed of a variable-conductance elastomeric composition. While functionally adequate in theory, such devices are relatively costly, and are less than satisfactory because of the difficulty in calibrating the devices and accurately and predictably measuring analog frequency or voltage variations. In addition, such known devices, in association with electronic circuitry for transmitting pressure data, have been plagued by difficulties inherent in the tire environment. Such difficulties include effectively and reliably coupling RF signals into and out of the tire, the rugged use the tire and electronic components are subjected to, as well as the possibility of deleterious effects on the tire from incorporation of the pressure transducer and electronics in a tire/wheel system. In the context of passive RF transponders that are powered by an external reader, another problem is generating predictable and stable voltage levels within the transponder so that the circuitry within the transponder can perform to its design specification.
It is further known to incorporate within surface acoustic wave (SAW) devices a programmable reflection pulse sequence by adding several sets of additional conductive stripes and then selectively floating and grounding them. A surface acoustic wave is reflected by any acoustic impedance discontinuity that it encounters in its path. By grounding the inter-digitated transducer on the surface of the device (i.e. in the path of the SAW wave), a change in the impedance discontinuity occurs and the acoustic reflection is increased. By electrically floating the inter-digitated transducer, the acoustic impedance discontinuity is reduced minimizing the acoustic reflection. Such devices are powered by an RF signal; data is stored on a read only memory (ROM), and the system is placed in position to be interrogated.
Such devices may have application, for example, in a high-speed rail traffic control system where a passing train interrogates a SAW device in the track as it passes. The reflection parameter from the SAW device is controlled by inputting the data stored on the ROM onto the reflection transducers. Thus, the reflection parameter signal is modulated as a function of time and the data is transferred thereby from the ROM via the SAW device to the train receiver. The data may describe the location and other information from the passive SAW device system to the train as it passes.